The increase in the popularity of soft contact lenses has led to many proposals for their manufacture. Prior art patents are replete with suggestions for various processing procedures for improving both the contact lens products and the manufacture thereof. The type and characteristics of the molds utilized in such manufacturing operations have received attention, since their ability to function in the desired manner as well as their cost are important commercial factors. However, in most prior processes the polymeric material used to fabricate the lenses and the processing operations used to form and finish the lenses are of primary importance.
It has been recognized that for the manufacture of soft contact lenses a casting process would be most advantageous. In the spin casting process as taught in U.S. Pat. No. 3,408,429, U.S. Pat. No. 3,496,254 and U.S. Pat. No. 3,660,545 the polymerizing mixture is contained in a rotating open mold having a concave surface. The anterior convex surface of the lens is thus formed by the mold surface, and the posterior, concave surface of the lens is formed as a result of centrifugal forces, surface tension of the polymerizing mixture, and other factors. The concave surface of the lens thus formed is approximately parabolic in shape and the operation must be carefully controlled to achieve reproducible shapes.
Other casting processes are based on the casting of contact lenses in closed molds. The prior art methods are primarily directed towards overcoming the lens quality problems which result from polymerization shrinkage and other variables in the molding process. With most monomeric materials the volumetric shrinkage on polymerization is in the range of 12 to 22%. For example, as taught in U.S. Pat. No. 3,660,545 (cols. 1 and 2) a polymerizing methacrylate ester mixture held in a closed glass mold invariably would pull away from at least one mold surface and cause the formation of surface voids which rendered the cast object unsuitable as a lens.
In the prior art method discussed in U.S. Pat. No. 3,660,545 the polymerizing mixture was held between concave and convex glass mold members having an annular gap interposed between them which decreased as polymerization occurred. An irregular edge configuration resulted from shrinkage which edge portion could be removed by cutting.
A further prior art method disclosed in U.S. Pat. Nos. 4,208,364 and 4,121,896 teaches the use of a mold comprising a male portion, a female portion, and a flexible rim portion. The male portion comprises one molding surface and the female portion similarly comprises a second molding surface. In the preferred embodiments both the male and the female portions each has a cylindrical support segment. When the mold is closed the male cylindrical segment fits into the female cylindrical segment. The flexible rim is attached circumferentially around one of the molding surfaces. During operation of the process the polymerization material is placed in the concave part of the mold and the male portion is placed into the female portion in such a manner that the tip of the flexible rim just touches the opposite molding surface. During the molding stage the molding material will contract as much as 20% and a potential vacuum is formed in the mold cavity. Under the external atmospheric pressure the two mold portions will move towards each other which movement is permitted by the flexibility of the rim. After polymerization the mold portions are separated and the lens produced stripped out from the mold.
When the rim is flexed the tip of the rim moves in a radial direction relative the axis of the lens. This movement is, however, far from predictable or uniform, since it depends on how much a given segment of the tip adheres to the mold surface. Also the movement may result in damage to the gel structure of the polymer and it is a normal part of the process that the edges have to be polished. Thus, the process has the usual disadvantage resulting from the necessity of finishing the lenses individually. A further disadvantage of the mold construction is that with certain lens configurations especially of minus power and with high water content the central portion of the lens tends to have poor optical properties.
Another mold construction is disclosed in U.S. Pat. No. 4,209,289. The mold having continuous mating surfaces with the parting line at the junction of the lens edge and anterior lens surface. An improved version of this mold construction is disclosed in U.S. Pat. No. 4,284,399; the improvement being that a groove is cut in the mating surface of the concave mold member. The area of said mating surface is thus reduced. It is stated that by proper choice of softening point temperature of the mold material, curing temperature of the contact lens material, and other factors such as dimensions of mating surfaces and weight placed on top of the mold, mold thickness in the area of the cavity and flexibility of the mold material, pre-release and distortion during cure can be controlled, and a lens having a finished edge can be cast. Polypropylene is the preferred mold material, and the mold thickness behind the concave and convex molding surfaces can range from 0.015 to 0.045 inch (0.38 to 1.14 mm). This mold construction is most suitable for casting of xerogel lenses (monomers without solvent or diluent) having polymerization shrinkage of 10 to 20% by volume.
In U.S. Pat. No. 4,197,266 a mold construction is disclosed which comprises first and second members each having a mold surface. Furthermore, the assembled mold also includes an annular reservoir which surrounds the mold cavity. The reservoir is connected to the mold cavity via a gap which is defined by co-operating aligning surfaces on the first and second mold member. In operation, the monomer held in the reservoir is held in an unpolymerized state and allowed to flow into the mold cavity via the gap during the polymerization. Any monomers which polymerize within the gap area may be subsequently removed from the lens by an edge contouring step.
The problems associated with shrinkage occuring during polymerization are recognized in U.S. Pat. No. 4,211,384 teaches utilizing a mold having a flexible gasket that deforms during polymerization thereby permitting the mold surfaces to be pressed together.
Recently issued U.S. Pat. No. 4,347,198 also addresses the shrinkage problem caused by polymerization. In this patent the disclosed process has the disadvantage of requiring an autoclave in order to avoid formation of voids and hollows. Although this patent discloses that the mold may be made from plastic, glass or metal, the preferred material is stated to be glass. All of the examples appear to be directed to the use of glass molds, which along with metal molds are prohibitively expensive. Despite the obvious advantages of lower costs, this patent fails to teach the art how to use plastic molds for this purpose.
There is a further requirement in this patent which calls for using very large amounts of solvent in the polymerizable mixture. This is required because of the limitations on polymerization shrinkage which can be handled in process. As an example: Whereas pure HEMA monomer will shrink 21-22%, the patented process handle 5% shrinkage during polymerization at the most, and in practice 3.5% isk the upper limit. In order to prepare a lens from HEMA it is therefore necessary to add sufficient solvent to bring the shrinkage of the polymerizing mixture down below 3.5% which means that the original solution will contain about 20% HEMA and 80% solvent. When after polymerization the lens precurser is soaked in water in order to replace the solvent with water, a considerable shrinkage will take place since the equilibrium water content of a poly-HEMA lens is about 40%. Such shrinkage is highly undesirable especially in an automated highly productive lens production since it introduces a source of variations and inaccuracies due to inhomogeneous and variable shrinkage of the lenses following the casting.